Two selenocysteine tRNA isoacceptors have been identified in mammalian cells in this laboratory. These tRNAs have two functions: 1) to read the termination codon, UGA, in protein synthesis and thus donate selenocysteine to the growing polypeptide chain in response to specific UGA codons; and 2) to serve as carrier molecules for the biosynthesis of selenocysteine. The pathway for the biosynthesis of selenocysteine is tRNA[Ser]Sec --- > seryl-tRNA[Ser]Sec --- > phosphoseryl-tRNA[Ser]Sec --- > selenocysteyl-tRNA[Ser]Sec. Both isoacceptors (designated NCA and CmCA on the basis of their anticodon sequences) arise from a single copy gene and differ from each other by five pyrimidine transitions which occur in the 5' half of the molecule. Therefore, one isoacceptor must be an edited version of the other. Both isoacceptors can be generated from the selenocysteine tRNA gene after its injection into Xenopus oocytes. Isolation of the primary transcript from Xenopus oocytes and its reinjection results in formation of both isoacceptors. We are presently investigating the pathway of synthesis of these isoacceptors as to whether one is a precursor of the other or both arise from a common precursor. The relative abundance of the selenocysteine tRNA population and the distributions of NCA and CmCA are influenced by the presence of selenium in mammalian cells grown in culture and in rat tissues taken from rats on diets with and without selenium, suggesting that tRNA editing occurs and the editing process responds to selenium. A selenocysteine tRNA that decodes UGA has been identified in animals, plants and protists in this laboratory (and in bacteria by other investigators) and thus, the universal genetic code has been expanded to include selenocysteine as the 21st naturally occurring amino acid.